This application is the U.S. national-phase application of PCT International Application No. PCT/FR98/02475.
Field of invention consists in treatment, separation and cleaning of liquids. Videlicet, the present invention relates to an apparatus for membrane separation that treats liquids by extracting suspended, emulsified and/or dissolved substances (mineral and organic ones) out of the said liquids. The same relates to a system and process of liquid separation.
The apparatus for membrane separation allows to obtain two flows of liquids such as:
a) flow of permeate or filtrate fully or partly free from suspended, emulsified and or dissolved substances (mineral and organic ones). This flow represents the major part of the liquid being treated;
b) flow of concentrate or retentate enriched with suspended, emulsified and or dissolved substances.
The present invention also relates to treatment of liquids containing chemicals, microbiological and pharmaceutical substances, foodstuffs that shall be efficiently removed or concentrated fully and/or selectively. In this field, the apparatus, system and process according to the present invention may have special advantages owing to a long-term serviceability without clogging membrane or with low clogging, maintaining a high permeability, not requiring a heavy pre-treatment of the said liquid being treated.
To avoid any misunderstanding of the present invention both itself and in comparison with the current state of the art, the following terms deserve explanation to be properly understood throughout this description.
xe2x80x9cA liquid being treatedxe2x80x9dxe2x80x94a liquid mainly comprising water or organics, containing organic or inorganic substances in whatever condition: suspended and/or dissolved and/or emulsified. This liquid is introduced into system for treatment.
xe2x80x9cA liquidxe2x80x9dxe2x80x94a liquid in the process of treatment within apparatus for membrane separation. It is obtained from the liquid being treated during treatment stage that is referred to as xe2x80x9cperiod of concentrationxe2x80x9d.
xe2x80x9cA concentrate or retentatexe2x80x9dxe2x80x94a part of liquid that, once failed to pass membrane, is enriched with substances retained on membrane until a required level of concentration is reached. As the required level of concentration is reached, the concentrate is let out whilst keeping its continuous level of concentration. Another stage of treatment referred to as xe2x80x9ca separation periodxe2x80x9d would commence as from this moment.
xe2x80x9cA permeate or filtratexe2x80x9dxe2x80x94a part of a liquid that, on passing a membrane, is fully or partly free from substances (colloidal, emulsified and/or dissolved ones).
xe2x80x9cConcentration level Tcxe2x80x9d within the period of concentration of the liquid being treated is determined by the following formula:       T    c    =                              V          p                                      V            p                    +                      V            cc                    +                      V            c                              ·      100        ⁢    %  
where
Vp stands for the permeate volume;
Vcc stands for the volume of a liquid in concentration tanks;
Vc stands for the volume of a liquid in pipes.
Once the required level of concentration is reached (separation commencement moment), Tc is determined by the following formula:             T      c        =                                        Permeate            ⁢                          xe2x80x83                        ⁢            flow            ⁢                          xe2x80x83                        ⁢            rate                                                                                                    xe2x80x83                                    ⁢                                                            Permeate                      ⁢                                              xe2x80x83                                            ⁢                      flow                      ⁢                                              xe2x80x83                                            ⁢                      rate                                        +                                                                                                                        Flow                  ⁢                                      xe2x80x83                                    ⁢                  rate                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  concentrate                  ⁢                                      xe2x80x83                                    ⁢                  being                  ⁢                                      xe2x80x83                                    ⁢                  bled                  ⁢                                      xe2x80x83                                    ⁢                  down                                                                    ·        100            ⁢      %        ⁢      xe2x80x83    "AutoRightMatch"
xe2x80x9cA membrane or filtering mediumxe2x80x9dxe2x80x94a filtering or separating medium aimed at a more or less selective, partial or full extraction of substances contained in a liquid i.e. colloidal, emulsified and/or dissolved ones. A membrane would normally comprise the two following layers:
the first one is a selective layer containing fine pores facing a compartment filled with a liquid and playing a major part in the separation;
the second one is a supporting layer facing a compartment filled with a permeate or filtrate. It aims at supporting the thin selective layer by imparting mechanical stability to it.
The present invention may use various types of membranes:
from the viewpoint of their use: for micro-filtration, ultra-filtration, nano-filtration and/or hyperfiltration at low pressure;
from the viewpoint of their composition: polymer, metal, ceramic and/or metal-ceramic filters and/or filters made of other suitable materials;
from the viewpoint of their structure: symmetrical, asymmetrical and composite filters.
xe2x80x9cAn operating clearancexe2x80x9dxe2x80x94a space between the main surface of a rotating body and the membrane surface.
xe2x80x9cA cellxe2x80x9dxe2x80x94a part of the apparatus for membrane separation, preferably of cylindrical shape, with a diameter longer than its length and representing a rotating body between two membranes with operating clearances on both its sides and bordered by a membrane on each side thereof.
xe2x80x9cA permeate chamberxe2x80x9dxe2x80x94a part of the apparatus for membrane separation, preferably of cylindrical shape, with a diameter significantly longer than its length and containing at least one substrate covered with a membrane on each of its main sides and used to support the membrane, to let in and out a permeate.
xe2x80x9cPermeabilityxe2x80x9d is determined by the ratio of permeate flow rate to the area of a membrane it has passed through.
xe2x80x9cOscillatory conditionsxe2x80x9dxe2x80x94a condition of a liquid characterized by transient and, if applicable, cyclic fluctuations of speed, flow rate and/or pressure of the said liquid under impact of controllable external forces.
xe2x80x9cCloggingxe2x80x9dxe2x80x94a phenomenon that restricts efficiency of any apparatus for membrane separation and reduces its permeability; this phenomenon consists in the following: deposition of a layer of substances on the membrane selective layer; plugging of membrane pores with the said substances; deposition of a polarization layer in the course of treatment of solutions containing salts and/or dissolved macromolecules.
xe2x80x9cCoarse pre-filtrationxe2x80x9dxe2x80x94pre-treatment of a liquid being treated for the purpose of extracting large particle representing a part of a suspended substance and having a size no less than 20 micrometers with the use of the traditional filtration technique.
By present, certain engineering solutions have already been made. For example, well-known is [document 1: Nakao S.-I. xe2x80x9cCurrent status of inorganic membrane in Japanxe2x80x9d. Proc. 2nd Inter. Conf. Inorg. Membr. -ICIM2-91, Montpellier, France, Jul. 1-4, 1991. Eds.: A. J. Burggraaff, J. Charpin and L. Cot. Trans Tech Publications published in xe2x80x9cKey Engineering Materialsxe2x80x9d, Vol. 61and62 (1991) pp. 219-228] a apparatus for membrane separation having flat rotating membranes. Soy-bean sauce micro-filtration does not result in filtration inferior of a certain minimum threshold of pressure due to centrifugal forces impacting on membrane permeability.
Apart from this, document 2: WO-A-95/09818 proposed a process of waste water treatment with the use of a complicated system including stages of physical-chemical deposition, micro-filtration and tangential nano-filtration. The above sequence of stages extends life of micro- and nano-filtration membranes owing to the sequence of various treatment processes: precipitation preceding micro-filtration, micro-filtration itself preceding nano-filtration. Apart from this, to avoid premature clogging of nano-filtration membranes, differential operating pressure of the above membranes was limited by 1.5 bar. However, it is well known that the micro-filtration stage that has to help avoid the nano-filtration membrane clogging will itself constitute a stage to cause a clogging problem.
Another invention (document 3: WO-A-96/09986) relates to processes and installation intended to treat liquids containing organic waste. The treatment chain includes stages of physical and chemical treatment, as well as ultra- or micro-filtration and then reverse osmosis. All stages of membrane-assisted treatment are of the classical type i.e. tangential stages. According to this invention, all stages of pre-treatment (pre-filtration, coagulation, flocculation, oxidation, filtration etc.) enable to extend the life of membranes at all stages of membrane-aided treatment. The system based on the two last inventions are very bulky and complicated.
Filtration apparatuses with a rotating body were described in the recent patents WO-A-95/00231, WO-A-96/01676, WO-A-95/16508, WO-A-92/21425 and U.S. Pat. No. 5,143,630 (documents 4 through 8 respectively). All these patents use rotating bodies adjacent to membranes and intended to reduce clogging thereof.
Apparatuses described in these patents (documents 4 through 8) are intended to reduce membrane replacement time. It is proposed to use modules containing membranes with own substrates. The above modules, though complicated in terms of composition, may easily and quickly be attached to filtration system and detached from the same.
On the other hand, these apparatuses are mechanically complicated and experience problems of wear, bulkiness and adjustment particularly as it concerns their rotating parts. They frequently require a hollow shaft, which makes the entire system more complicated and less robust. These inventions have led to the solution of the membrane clogging problem that may be characterized as partial in comparison with inventions using classical filtration with tangential flow.
Subject of Document 6: WO-A-95/16508 consist in a shaft that fastens rotating bodies of the apparatus for membrane separation. It was proposed to apply a rotating body having at least two blades fixed on the said shaft by means of rods. When ready to work, these blades would assume a radial i.e. open position. To replace membranes, the blades are folded, which allows to easily remove the entire membrane module.
In Document 6, the increased diameter of the shaft with folded blades reduces the active area of a membrane. In particular, the D/d ratio, i.e. the ratio of diameter D of open blades to shaft diameter d with folded rotating bodies would range only from 2.1 to 2.6. The rotating shaft makes the balancing of this assembly highly problematic. The shape of blades is determined by requirements for their maximum compactness when folded.
A number of already mentioned patents (Document 7: WO 92/21425 and Document 8: U.S. Pat. No. 5,143,630) propose to vibrate rotating bodies distributed over the shaft so that to reduce membrane clogging. However, no engineering solution has been found as to how such shaking movements may be performed.
Proceeding from the above study of state of the art, one may conclude that the efforts were primarily focused on facilitating replacement of modules containing membranes without removing shaft carrying rotating bodies. Such apparatuses have many drawbacks: the internal composition of the apparatus for membrane separation is more complicated; it is more difficult to maintain impermeability, it is impossible to operate at high pressure. All the above engineering solutions have only been proposed due to membrane clogging and the necessity of replacing them frequently. It means that the problem of membrane clogging reduction still awaits its more efficient solution. An essential reduction of this clogging would reduce the frequency of membrane replacement significantly lower, which would allow to simplify apparatus for membrane separation composition.
The purpose of present invention is to propose an apparatus for membrane separation with a lower clogging, that comprises four flat fixed membranes, at least three bodies rotating next to the selective layer of the above membranes and generating secondary vortex, plus at least one apparatus that generates oscillations in the liquid.
Another purpose of the present invention is to propose an apparatus for membrane separation, wherein each membrane is subjected to vibration so that to avoid or delay its clogging. The above apparatus has at least four membranes resting upon two semi-stationary substrates of a strong and simple structure, each comprising a single disk made preferably of baked metal powder or another made of suitable porous material. The above disk covered with membranes on either side remains vibrating due to oscillatory fluctuations of flow and liquid pressure on the one hand and continuous rotation of bodies fixed on shaft on the other hand. Superposition of membrane vibration and liquid flow oscillation facilitates reduction of membrane clogging.
One more purpose of the present invention is to extend separating system serviceability between membrane washing, as well as to improve properties of used membranes. The proposed structure of rotating body having an impeller shape allows to evenly distribute load of substances retained by the membrane over its entire surface. Such an even distribution allows, on the one hand, to improve membrane efficiency and clogging on, the other hand, to avoid formation of stagnant zones apt to heavier.
Another purpose of the present invention is to increase load on membrane of suspended substances that exceeds values determinable by method xe2x80x9cSilt Density Indexxe2x80x94index de la densitxc3xa9 de dxc3xa9pxc3x4txe2x80x9d (see Document 9: ASTM D 4189-82), without reduction of membrane operating efficiency. This is a very significant advantage since up to the present it has not been possible to treat such liquids by previous filtration methods without a thorough pre-treatment whatever the filtration method used (spiral modules, tangential method and/or method with rotating disks in the liquid medium moving evenly i.e. in the non-oscillatory manner). This was checked both for suspended and dissolved substances. In general, the present invention makes it possible to use ultra- and nano-filtration and reverse osmosis for treating liquids, where the concentration of suspended, dissolved and/or emulsified substances exceeds values admissible for the previous methods, and all that without pre-treatment of the above liquids by means of micro-filtration.
One more purpose of the present invention is to offer an all-purpose system of liquid separation that may be used for micro- ultra- and/or nano-filtration, as well as reverse osmosis at low pressure. In all the above cases, one may use the liquid either without any pre-treatment or with an insignificant pre-treatment (a single stage of pre-filtration). In the last case, pre-treatment is used to remove large particles from the liquid being treated, which intends not reduction of clogging risk but avoidance of a course mechanical impact capable of injuring selective layer of membrane.
One more purpose of the present invention is to offer a process of liquid separation with the use of the aforementioned apparatus that may be tailored to various specific restrictions required. In particular, the same apparatus and the same separation system may be used for the following processes: micro-, ultra-, nano-filtration and reverse osmosis by mere use of appropriate membranes. Thanks to enhanced resistance to clogging, the separation process may be offered, on the one hand, for reaching higher concentrations of the component in hand in the concentrate and, on the other hand, for the treatment of highly concentrated liquids. A large number of liquids being treated no longer need additional chemical agents to retard membrane clogging and thus to extend their serviceability until next washing. Absence of pre-treatment stages makes separation systems simpler from the viewpoint of their application and more compact.
One more purpose of the present invention is to increase reachable specific permeability of the membrane thanks to prevention or reduction of liquid components deposition on the membrane selective layer. Furthermore and for the same reason, the membranes will retain their high permeability for a longer period. In general, both these conditions enhance apparatus for membrane separation efficiency.
Purposes set forth above, as well as others that will be dealt with below, have been reached, as per the invention, thanks to an apparatus for membrane separation intended to separate a component (components) contained in the liquid being treated. A distinctive feature of the apparatus for membrane separation is that it comprises a fixed housing with axial symmetry, at least four flat membranes having the shape of disks with a central opening to enter a shaft that holds at least three bodies in the immediate proximity to the above membranes. The said shaft being in rotary motion drives the aforementioned bodies. The above mentioned housing contains permeate chambers interspersed with cells and crossed with a shaft. The latter is driven by a motor or any other apparatus by means of gears. Each cell comprises a body put on the shaft and two spaces left between the aforementioned body and surfaces of membranes located on either side of the abovementioned rotating body. The said spaces constitute operating clearances. The width of operating clearance may be changed between 0.5 and 50 mm, preferably between 1 and 6 mm. The apparatus for membrane separation contains two types of cells: no less than one intermediate cell, each of the two sides of this cell comprises a membrane, and of at least two extreme cells, one side of these extreme cells formed by a membrane, whereas another side formed by the end wall of the cylinder-shaped housing, the aforementioned first end wall containing at least one common liquid inlet means (i.e. an extreme inlet cell), and the aforementioned second end wall containing at least one common liquid outlet means (i.e. an extreme outlet cell). The aforementioned second end wall also includes a shaft fastener. Membranes divide the apparatus for membrane separation into two compartments: the first one containing the liquid, whereas the second one containing the permeate.
The common liquid inlet and outlet means are located on opposite end walls between the shaft and the outer edge of these walls. The liquid may be additionally let out from the first compartment of the apparatus for membrane separation through at least one peripheral liquid outlet means located in the ring wall of each intermediate cell. The ratio of the liquid flow rate through the common liquid outlet means to the sum of the liquid flow rates through all peripheral liquid outlet means determines the ratio between the sequential and parallel flows distributed among the cells of the apparatus for membrane separation. A part of the liquid that passed membrane i.e. permeate fills a part of the apparatus for membrane separation referred to as a permeate chamber and is let out over the outer edge of the chamber through at least one permeate outlet means.
Rotating bodies put on the shaft are located in each cell of the apparatus for membrane separation. These bodies, mainly shaped as an impeller, are fitted with at least two blades connected with the central ring of the above impeller. Each of the aforementioned blades has two main surfaces whose curvature along circumference line may be negative, zero or positive. The curvature of main surfaces of impeller blades located in an intermediate cell is mainly zero. The curvature of main surfaces of blades located in an extreme cell is mainly positive (i.e. convex) with respect to the surface located opposite to the common liquid inlet means (or outlet means), and zero with respect to the surface located opposite to the membrane. The above surfaces are limited either by front and rear sharp edges bent in the form of spiral, or by the circumference edge or outer edge. Side sharp edges bent in the form of spiral allow to reduce energy losses when the impeller is in rotation. The circumference edge coincides with the main axis of shaft, and the impeller having such an edge is located mainly in each extreme cell. When the outer edge does not coincide with main axis of shaft, the impeller having such an edge is located mainly in the intermediate cell. Geometry of front and rear edges is calculated by the formula stated below.
Let us assume that N=kxcfx80 is the angle of contact of each of two side edges that determine the shape of each impeller blade (kxe2x80x94coefficient). Thus, the shape of front and rear edges is described by the following formula:                               χ          ·                      sin            ⁡                          (                              χ                +                                  m                  ·                                      π                    n                                                              )                                      =                  χ          ·                      cos            ⁡                          (                              χ                +                                  m                  ·                                      π                    n                                                              )                                                          (        I        )            
where
"khgr" stands for a current angle of front or rear edges of the blade;
m stands for an integer number that determines (in xcfx80/n units) the original angle of the edge relative to the horizontal axis (abscissa) protruding from the center of a circle circumscribed by blades and constituting the edge of the circumference of each impeller;
n stands for number of blades.
In equation (1), absolute value of "khgr" ranges between 0 and N. According to the present invention, coefficient k ranges between 0.05 and 1, preferably between 0.1 and 0.6.
Number of blades may range between 2 and 12. In the same manner, the angle between front and rear edges of each blade ranges between 15 and 180xc2x0, whereas the angle between the front edge of one blade and the rear edge of the next blade may vary between 0 and 165xc2x0. The ratio of the radius of blades circumference to the radius of the outer edge of the central ring which supports these blades varies between 3 and 15.
Number of impeller revolutions ranges between 20 and 5,000 per minute, preferably between 200 and 2,500 per minute, and may be altered as desired in the course of the apparatus for membrane separation operation.
The phase of mutual position of impellers in intermediate cells may vary between 0 and 180xc2x0.
The impeller of the apparatus for membrane separation extreme cell contains blades with front and rear edges of the same shape as that of the impeller located in the intermediate cell. This cell may be calculated by equation (1). The liquid flow would diminish for a short time, as the blade passes a liquid inlet or outlet means. This takes place cyclically. Main surfaces of the aforementioned blades being mainly convex on the side of the said liquid inlet (or outlet) means allow to smoothly diminish flow of liquid passing these liquid inlet (or outlet) means and thus to avoid a hydraulic shock. The proposed shape of blades oscillates the liquid, acting upon its flow rate (i.e., upon its linear speed) of this liquid in the apparatus for membrane separation. To improve the effect of removal of suspended substances from the zone adjacent to the membrane surface in the extreme cell, the curvature of the main surface facing the membrane may be reduced in comparison with the curvature of the opposite surface. The phase of impeller mutual position in each extreme cell may vary between 0 and 180xc2x0.
To excite oscillations in each intermediate cell of the apparatus for membrane separation, it is also proposed to use another design of the impeller containing blades circumscribing the outer edge with radius Rp, whose length is less than the length circumscribed by radius R between the central axis and the extreme point of the said edge located at the longest distance from the aforementioned central axis. The above point makes a circle with the said radius during the rotation of the impeller referred to above. Each blade of the said impeller comprises two main surfaces positioned opposite to appropriate membranes; these surfaces are bordered by front and rear sharp edges bent mainly in the form of spiral in accordance with equation (1). The aforementioned outer edge has the form of an arc which does not coincide with the main axis of the shaft being the same as the axis of the apparatus for membrane separation housing. The aforementioned radius Rp has a reference point resting mainly on the median of an arc that forms the aforementioned outer edge of the impeller blade. This median crosses the impeller axis of rotation. Since radius Rp shall always be shorter than radius R, the curvature of the impeller blade outer edge remains larger that the curvature of the circle embracing blades and circumscribed by the extreme point of the blade outer edge during the impeller rotation. Rp/R ratio lies between 0.1 and 0.99, preferably between 0.7 and 0.95.
The intermediate cell is fitted mainly with at least one liquid peripheral liquid outlet means fixed on the ring wall of this cell whose main axis forms an angle with the center axis opening in the above ring wall, lying between 0 and 90xc2x0. The aforementioned peripheral liquid outlet means having the above angle is tilted mainly in the direction of the impeller rotation. The opening in the above outlet means fixed on the ring wall will be adjacent to the aforementioned outer edge of the blade.
Flow rate in the intermediate cell will increase or decrease cyclically as the impeller set on a solid or hollow shaft rotates around its axis, and allows to obtain oscillating liquid flow in each intermediate cell of the apparatus for membrane separation. To have the apparatus for membrane separation operate properly, the number of blades of the last impeller design shall be equal mainly to the number of liquid peripheral liquid outlet means, provided that the number of these liquid outlet means exceeds one. The angle between extremum lines to the outer edge of each blade shall be equal to the angle formed by lines between peripheral liquid outlet means opening centers. If these conditions are complied with, oscillations of liquid flow in this cell are subject to cyclic law whose frequency equals to the frequency of impeller rotational movement multiplied by the number of blades.
Blades continuously moving in the immediate proximity to the selective layer of membranes and having edges bent in the form of a spiral are intended:
A. To cause rotational movement of liquid by imparting to the latter a high speed in the operating clearance and making this speed more even over the entire surface of a membrane;
B. To reduce loss of energy in the above operating clearance;
C. To serve as a means of exciting oscillations of liquid (two extreme impellers) at the apparatus for membrane separation inlet and outlet. These two extreme impellers may have either the same or a different number of blades in comparison with intermediate impellers of the apparatus for membrane separation and in comparison with one another. The first main surface of the above extreme impellers, which is located opposite to the membrane, is required to prevent its clogging. The second main surface of the above extreme impellers is required to excite oscillations of the liquid, while the blades themselves are required to interrupt jets of this liquid. The impeller rotation speed determines the frequency of oscillations, whereas the number and width of blades at the level of a radius coinciding with the axis of liquid liquid inlet and outlet means determine the frequency and amplitude of oscillations at the same time. The aforementioned extreme impellers may be fixed on the apparatus for membrane separation shaft so that either to be or not to be in phase with one another. The distance between a liquid inlet and/or outlet means opening and the main surface of the blade will be either adjustable or continuous.
D. To serve as a means of exciting oscillations of membranes fixed on main surfaces of permeate chambers which have the shape of a thin disk. On each side of a porous substrate which represents the above chambers there is an impeller having at least two blades and rotating. If two impellers surrounding the same permeate chamber are not in phase, there is a local pressure gradient on either side of the said permeate chamber. The aforementioned gradient changes with impellers rotary movement and causes vibration of the said permeate chamber and membranes.
E. To serve as a means of exciting oscillations of liquid flow within the operating clearance by short-term and cyclic interruption of this liquid exit through at least one liquid outlet means located in the ring-shaped wall of each cell. The distance between the peripheral liquid outlet means opening and the extremum of blade outer edge is either variable or continuous.
The distance between the membrane surface and the main surface of the impeller blade (i.e. operating clearance) may be continuous along the radius and the circumference line of the impeller or may vary along these lines. In the last case, the angle between the membrane surface and the main surface of blade ranges between 0 and 30xc2x0.
Making the rotating body in the shape of an impeller containing at least two blades as compared to a flat disk or a disk with grooves or lugs (radial, concentric or spiral ones) gives a number of advantages: a) this decreases friction forces between the rotating body surface and liquid thus reducing energy losses and consequently reducing liquid warming in the course of apparatus for membrane separation operation retaining at the same time the required speed for efficient removal of a substance from the membrane surface by means of liquid flow; b) by affecting semi-stationary substrate, this supplements non-stationary cyclic movement with a second component which causes membrane vibration; c) this supplements non-stationary cyclic movement of liquid with a third component by means of partly short-term and cyclic closing of the liquid outlet means located in each ring-shaped wall of each apparatus for membrane separation cell. Combination of these non-stationary cyclic motions enables to reduce the speed of substance precipitation onto the membrane and/or to facilitate removal thereof from the membrane surface.
The permeate chamber comprises a porous substrate which includes at least one porous disk, main sides of which are covered with membranes normally made of a polymeric compound. The center of the disk has a opening used to pass a shaft and liquid around central rings of the impeller. The above substrate may also comprise a porous material fairly strong, of symmetrical, asymmetrical and/or composite type, main surface(s) of which is (are) covered with a selective layer. The porous disk may mainly be made of baked metal powder, be ceramic or ceramic metal. It may be covered with a selective layer of a polymeric compound and/or ceramic or ceramic metal bonded with the substrate. The size of porous disk pores varies from 1 to 500 microns whereas porosity percentage ranges between 5 and 80%. On the one hand, it is required to make permeate resistance to flow as low as possible, while on the other hand to prevent pores from injuring the structure and integrity of the membrane used even under a high differential pressure. At the same time porosity percentage of the above porous disk shall be optimal so that to ensure strength of the substrate, which shall be sufficient for specific operating conditions.
The permeate leaves the aforementioned chamber over its outer edge and through a part of its main surface adjacent to the outer edge. Then it may be collected in a tank or leave outwards through an apparatus installed in a case surrounding each chamber. The substrate has an inner edge coinciding with the above outer edge. The said inner edge forms a space used to insert the shaft. This inner edge and a part of membranes in touch with the above edge are sealed. Each of the aforementioned chambers is fixed by its inner circumference on the ring-shaped wall of adjacent chambers, thus forming the apparatus for membrane separation housing, and its central part is left free.
The function of a permeate chamber may be discharged by a single disk covered on each side with a membrane or selective layer bonded to the said sides. In this case, the permeate chamber will comprise pores located inside a porous disk; liquid enters this chamber through membranes, then it flows through pores inside the porous disk to outer edge, through which it leaves the apparatus for membrane separation. Experience of using a single porous disk as a permeate chamber may be used for processes of reverse osmosis and/or nano-filtration and/or sometimes ultra-filtration, i.e. for processes in which specific permeability of membranes is low.
In case of micro-filtration and/or sometimes ultra-filtration, when membrane permeability is higher, one may use at least two porous disks for the permeate chamber. To facilitate withdrawal of permeate to the outer edge of the aforementioned substrate, the said porous disks are separated by means of a net or, for instance, another porous disk. Such a net or an internal disk pore size is larger than that of pores of outer substrates. One also may form radial or concentric grooves on the side opposite to the membrane-covered side of each extreme disks, and/or on either side of the inner disk. In the last case, the permeate will mainly leave either over the outer edge of the net or the inner substrate having large pores, or, besides it, through the above grooves.
Each membrane used in the apparatus for membrane separation constitutes a disk having a opening in its center. There is no bonded between various membranes of each cell as well as between various pieces of the membrane which covers each side of the porous disk except for the outer edge, which prevents formation of stagnation zones which would normally occur around the above bonded points. Sealing gaskets located near outer and inner edges of the substrate are properly washed by liquid vortex flow generated by rotating bodies.
The ratio of the size of pores dm in the selective surface of the membrane used in the apparatus for membrane separation to the size of pores Ds of the substrate which constitutes the permeate chamber is as follows: Ds/dmxe2x89xa750. Permeate chamber thickness ranges between 0.5 and 10 mm, mainly between 1 and 5 mm.
The apparatus for membrane separation housing is in axial symmetry with respect to the shaft. The latter is mainly of solid or hollow material in its part located inside the apparatus for membrane separation and being in contact with liquid. In case of a hollow shaft, liquid enters the apparatus for membrane separation through the common liquid inlet means and/or axial inlet means, flows around and/or inside the shaft and fills various cells, as well as the operating clearance. The above hollow shaft contains at least one radial bore made in its wall at the level of each porous substrate and used to introduce liquid into each operating clearance. Each rotating body includes a central ring which has at least one bore used to introduce liquid into each operating clearance, too. The above bores in the central ring of the impeller coincide with bores made in walls of the hollow shaft. Each internal cell is fitted at least with a single peripheral liquid outlet means. Ends of the apparatus for membrane separation fitted with a hollow shaft contain extreme cells comprising an extreme rotating body and a single membrane on one side of this rotating body and a single liquid inlet (or outlet) means on the other side of this rotating body. Rotating bodies inside the aforementioned extreme cells even in the shape of an impeller have an extra purpose in comparison with the bodies located inside intermediate cells.
The liquid inlet means is located in the immediate proximity to the rotation plane of the aforementioned extreme impellers. Interruption of the above liquid flow by rotating blades excites oscillations of the liquid flow in the entire apparatus for membrane separation. Another impeller located in another extreme cell may have the same purpose. In the latter case, interruption of the liquid jet by rotating blades will also excite oscillations of the liquid flow in the entire apparatus for membrane separation. These two impellers located in the aforementioned extreme cells of the apparatus for membrane separation may be either in phase, anti-phase or have any other difference in terms of phase. Thus, there may be several ways of generating oscillatory conditions in liquid inside the apparatus for membrane separation.
Another source of liquid oscillation consists in the intermittent and cyclic discontinuation of the liquid flow which leaves through at least one peripheral liquid outlet means located in the ring-shaped wall surrounding each intermediate cell of the apparatus for membrane separation with a hollow shaft. Reduction of the curvature radius of the blade outer edge in comparison with the impeller circumference line curvature efficiently results in oscillatory conditions within the operating clearance of an appropriate cell.
Oscillatory conditions generated in the apparatus for membrane separation with a hollow shaft will influence the membranes used. Indeed, membranes rest on porous substrates having the shape of a disk with its thickness ranging from 0.5 to 10 mm, thus constituting a permeate chamber. Furthermore, the inner edge of the above chambers is not fixed, thus facilitating vibration of these permeate chambers and consequently of membranes which cover main sides of the above substrates. A wave generated in liquid under the impact of oscillations caused by at least one impeller of extreme cells will spread from an extreme cell of the apparatus for membrane separation toward another cell sequentially crossing each extreme cell of the apparatus for membrane separation. Consequently, there is a local gradient of speed and pressure on either side of each permeate chamber. This gradient changes cyclically and brings membranes into vibration.
Another source of membrane vibrations consists in the mutual position of impellers of various cells. Should there be a phase displacement of impellers surrounding the same permeate chamber, there is a local pressure gradient on either side of the above chamber. The said gradient changes cyclically in each point of a membrane and its substrate thus exciting vibration of the above membranes.
The aforementioned oscillatory conditions in liquid facilitate removal of liquid from membrane selective layers, thus enabling to ensure high specific permeability thereof for a long period of time. This means that it is possible to reduce the frequency of washing and replacement of membranes. The spiral shape of impeller blade edges allows, on the one hand, to eliminate deposition of particles on the membrane surface and, on the other hand, to make even the tangential speed over the entire membrane surface. Superposition of the aforementioned various effects which occur in the liquid will require permanent and intensive cleaning of membrane selective layers and efficiently retard clogging thereof.
The present invention allows to make a number of apparatus for membrane separations from the same cells and permeate chambers.
According to the first embodiment, the apparatus for membrane separation will include:
a fixed cylindrical housing separated by membranes into two compartments, the first filled with liquid and the second with permeate;
at least four flat membranes resting on two porous substrates on either side of the above substrates. These substrates have the shape of a disk with a opening in the center;
at least one common liquid inlet means of the first compartment. This liquid inlet means located in the first end wall of the above housing between the shaft and the outer edge of the wall is directly connected to the first extreme cell;
at least one common liquid outlet means of the first compartment. This liquid outlet means is located in the second end wall between the shaft and the outer edge of this end wall and/or in a ring-shaped wall which surrounds each intermediate cell;
at least one permeate outlet means in the second compartment. This outlet means is located on the outer edge of the above porous substrates;
at least three bodies located in the first compartment in the immediate proximity to the above membranes, thus forming operating clearances. Each of the said bodies includes a central ring put on:
a hollow shaft which constitutes the axis of the aforementioned housing inserted through a central opening made in the above membranes and substrates and being in rotational movement which causes rotation of these bodies, generating secondary vortexes and oscillatory conditions at the level of the above operating clearances, as well as resulting in oscillatory movement of membranes in order to avoid or reduce clogging and ensure high specific permeability thereof; and
an apparatus to cause rotation of this shaft bearing the said bodies.
Naturally, one may use the apparatus for membrane separation in this first embodiment by introducing liquid into means located in the second end wall and/or in the ring-shaped wall of each intermediate wall and by collecting it through the common apparatus located in the first end wall of the housing.
According to the second embodiment, the apparatus for membrane separation includes the following:
a fixed cylindrical housing separated by membranes into two compartments, the first one filled with liquid and the second one with permeate;
at least four flat porous fixed substrates on either side of the above substrates; these substrates have the shape of a disk with a opening in its center;
at least one axial liquid inlet means of the first compartment; the said liquid inlet means is located in the first end wall of the above housing on the end of the hollow shaft;
at least one liquid outlet means of the first compartment; the said outlet means is located in the second end wall between the shaft and the outer edge of this end wall and/or in the ring-shaped wall surrounding each intermediate cell;
at least one permeate outlet means in the second compartment; this outlet means is located in the outer edge of the aforementioned porous substrates;
at least three bodies installed in the first compartment in the immediate proximity to the above membranes, thus forming operating clearances; each of the above bodies includes a central ring; the said ring of each body has at least one radial bore used to introduce liquid into each of operating clearances; the above bodies are put on:
a hollow shaft constituting the axis of the aforementioned housing inserted into the central opening made in the aforementioned membranes and substrates and being in continuous rotational movement, which causes rotation of these bodies, generating secondary vortices and oscillatory conditions in the liquid, in the aforementioned operating clearances, as well as oscillatory movement of the membranes in order to avoid or reduce clogging and ensure high specific permeability thereof; the aforementioned hollow shaft comprises at least one radial bore made in the wall thereof, at the level of each porous substrate, and used to introduce the liquid into each operating clearance; the aforementioned bores of the central ring of the rotating bodies are coinciding with the hollow shaft bores; and
an apparatus to cause rotation of this shaft bearing the said bodies.
Naturally, one may use this second embodiment of the apparatus for membrane separation by introducing the liquid into means located in the second end wall and/or in the ring-shaped wall of each cell and by collecting it through the axial apparatus located in the first end wall of the housing on the hollow shaft end.
According to the third embodiment, the apparatus for membrane separation includes:
a fixed cylindrical housing separated by membranes into two compartments; the first one filled with liquid and the second one filled with permeate;
at least four flat membranes set on two porous substrates on either side of the above substrates. These substrates have the shape of a disk with a opening in the center;
at least two liquid inlet means of the first compartment, these liquid inlet means being located in the first end wall of the above housing; the first one of the aforementioned means being a common liquid inlet means of the first compartment, this first apparatus, located between the shaft and the outer edge of the end wall, being directly connected to the extreme liquid inlet cell; the second one of the aforementioned means being an axial liquid inlet means of the first compartment, this second apparatus being mounted on the end of the hollow shaft;
at least one liquid outlet means of the first compartment. This outlet means is located in the second end wall, between the shaft and the outer edge of this end wall, and/or in a ring-shaped wall which surrounds each intermediate cell;
at least one liquid outlet means in the second compartment to carry away the permeate produced from the said liquid. This liquid outlet means is located on the outer edge of the above porous substrates;
at least three bodies located in the first compartment in the immediate proximity to the above membranes thus forming operating clearances. Each of the said bodies includes a central ring; the said ring of each body having at least one radial bore to introduce the liquid into each operating clearance; the said bodies being set on:
a hollow shaft constituting an axis of the aforementioned housing, inserted into a central opening made in the aforementioned membranes and substrates and being in continuous rotational movement, which causes rotation of these bodies, generating secondary vortices and oscillatory conditions in the liquid, in the aforementioned operating clearances, as well as oscillatory movement of the membranes in order to avoid or reduce clogging and ensure high permeability thereof; the aforementioned hollow shaft comprises at least one radial bore made in the wall thereof, at the level of each porous substrate, and used to introduce the liquid into each operating clearance of the apparatus for membrane separation; the aforementioned bores of the central ring of the rotating bodies are coinciding with the hollow shaft bores; and
a apparatus to cause rotation of this shaft bearing the said bodies.
Naturally, one may use this third embodiment of the apparatus for membrane separation by introducing the liquid into the means located in the second end wall and/or in the ring-shaped wall of each cell and by collecting it through the axial and/or common liquid outlet means located in the first end wall of the housing.
In these three embodiments of the apparatus for membrane separation the fixed cylindrical housing and shaft are horizontal. The apparatus for membrane separation design compliant with any of the embodiments described above, but having a vertical cylindrical housing and shaft is also possible. In this vertical design the permeate collecting tank, shaft fastener and the common liquid inlet means can be mainly located in the lower end wall of the apparatus for membrane separation and the common outlet means and the axial inlet means of the said liquid are located in the upper end wall. This vertical design allows easier access to the membranes when they are installed and/or replaced.
According to the present invention, it is possible to mainly avoid the use of the hollow shaft through using blade-containing impellers. In this case the liquid enters the apparatus for membrane separation via a common liquid inlet means located in the first end wall of the cylindrical apparatus for membrane separation, goes around the shaft, using the space between the impeller blades for this purpose, also getting into each cell and hence into each operating clearance. The ratio of the flow going around the shaft to the flow passing through each clearance can be adjusted with the valves connected to the common liquid outlet means and to the peripheral liquid outlet means.
The purposes of the present invention have been achieved by superposing the rotary movements of the impeller-entrained liquid, the radial movements of the said liquid and finally the oscillatory movement of the said liquid on each other. These movement all superimpose on each other in the operating clearance. Thus, the said movements create, in the liquid situated in the immediate proximity to the selective layer of the membrane, the conditions which produce a positive effect, retarding or even delaying particle deposition on the membrane. Moreover, the aforementioned oscillations which exist in the liquid, cause the membrane itself to vibrate, thus preventing accumulation or even extracting the substances which have already penetrated the pores of the membrane. The aforementioned oscillatory conditions in the liquid, which cause the membrane to vibrate, can differ by their origin into (a) waves which propagate via the apparatus for membrane separation from cell to cell and which are generated by the rotating extreme impeller upstream of the common liquid inlet means and/or upstream of the common liquid outlet means; (b) waves generated in each cell by the bladed impeller, whose blades cyclically turn on the liquid flow in each cell for a short period of time; the volume of propagation of the latter waves being limited to the volume of the particular cell.
The oscillatory conditions in the liquid in the operating clearance of the apparatus for membrane separation, as well as membrane vibrations help:
to change the transition between the laminar and turbulent conditions in the said clearance;
the suspended substance to migrate and the (macro)molecules to diffuse from the selective layer of the membrane toward the operating clearance;
to change the liquid speed cross-section existing in the operating clearance of each cell so as to bring the fastest layers of this liquid close to the selective layer of the membrane; this effect results in a substantial increase of the speed gradient existing in the immediate proximity to the selective layer of the membrane, thus retarding the process of clogging.
The operation of the apparatus for membrane separation proposed in the present invention involves little or no flushing. Optimization of the hydrostatic (pressure) and hydrodynamic (apparatus for membrane separation liquid flow rate, operating clearance speed components) parameters, the oscillation frequency and amplitude of each liquid being treated and the membrane vibration parameters used to treat a particular liquid help maintain the membrane highly permeable for a long period of time. Generally, there is an amplitude minimum (expressed in terms of the amount of the liquid transported per oscillation period); when it is exceeded, the positive effect on the permeability and temporal behavior of the membrane ceases to exist. This minimum is equal to one tenth of the amount of the liquid contained in each operating clearance. The liquid frequency shall be in the range between 0.1 and 1,000 Hz, preferably between 1 and 400 Hz. Various cyclic actions, which are simultaneously excited in the apparatus for membrane separation and each cell thereof shall preferably have other-than-quadrature phase displacement in respect to each other to prevent stagnant zones from forming.
To cause the liquid in the apparatus for membrane separation to oscillate, use can be made of a bladed impeller, valves, pumps and other piston means, peristaltic pumps. The said valves can be installed upstream or downstream of the apparatus for membrane separation, the said pumps are generally installed upstream of the apparatus for membrane separation.
According to the invention, the concentration polarization associated with reverse osmosis, nano- and ultra-filtration can be further decreased, absorption layer development and/or pore clogging associated with the processes of micro- and ultra-filtration can be reduced or prevented by the use of an electric field which is applied on either side of each membrane of the apparatus for membrane separation, for which purpose the impeller and the metal porous substrate of the membrane are used as the opposite electrodes. The said electrodes are coated preferably with silver and/or platinum. The electric field voltage can be direct-current or alternating-current voltage or, besides, it can be pulse voltage. It is preferable to apply a DC electric field whose intensity is intermittently varied in time, the field voltage being constant for a pre-selected period of time and then decreased to the minimum or even to zero and restored at the end of this xe2x80x9cdead periodxe2x80x9d.
There is the minimum threshold of the electric field voltage which is generally equal to the force of xe2x80x9cconvectionxe2x80x9d existing in each operating clearance by virtue of the differential pressure applied on either side of the membrane. The said force removes the components which cannot pass through the membrane pores and thus become deposited on its selective layer. The other possible cause of existence of the said threshold is the electric resistance of the operating clearance and filtering medium. To have the best apparatus for membrane separation performance, use shall be made of an electric field whose voltage is greater than the threshold. According to the present invention, the DC field voltage is beyond the aforementioned threshold, ranging between 500 and 50,000 V/m. The ratio of the duration of the above voltage to the duration of the dead period is in the range between 0.1 and 50.
Another subject of the present invention consists in a separation system, which includes:
an apparatus for membrane separation
at least one apparatus for building up differential pressure on either side of the membrane; the said apparatus being located in the pipeline for letting in the liquid into the apparatus for membrane separation, upstream of the said apparatus for membrane separation and/or in the permeate pipe;
at least one apparatus for controlling the pressure and flow rate, located in the liquid circulation pipeline, downstream of the apparatus for membrane separation;
a tank for concentrating the liquid being treated, collecting the permeate, letting out the concentrate, keeping the detergent solution;
an apparatus for dispensing the liquid into the cells in the sequential or parallel manner or using a combination thereof;
an apparatus for optimizing the hydrostatic (pressure) and hydrodynamic [apparatus for membrane separation liquid flow rate, (rotary and radial) components of the speed in each operating clearance] parameters; the aforementioned means can be a geared motor, intermediate and extreme impellers, valves, pumps;
an apparatus for controlling the concentrate/permeate flow rate ratio; such apparatuses can be provided by, e.g., electric-signal flow meters;
at least one heat exchanger located in the liquid circulation loop and/or at least one heat exchanger located in the concentration tank; these heat exchangers being intended for stabilizing the liquid temperature;
at least one subsystem for flushing the apparatus for membrane separation;
at least one subsystem for pre-treating the liquid before it is introduced into the aforementioned apparatus for membrane separation;
an apparatus for controlling and/or recording the liquid and operating characteristics.
In the apparatus for membrane separation the liquid is prepared in the concentration tank of the liquid being treated by removing the permeate from the said liquid. The permeate is piped into the permeate collecting tank. The introduction of the liquid being treated into the concentration tank is preferably via a level controller. This step is a step of concentration of the liquid being treated. Having a concentration tank allows the system to operate continuously, handling a large amount of the liquid being treated. Of course, this system can be used to handle the liquid being treated in an intermittent manner, in batches, whose volume is equal to that of the concentration tank. The aforementioned concentration step is characterized by that the concentrate outlet valve is closed. The liquid is concentrated by pumping it under pressure through the concentration loop. This concentration step lasts until the desired level of concentration is reached. Once the said level of concentration is reached, the concentrate outlet valve is opened and the concentrate outlet pump, used to remove the concentrate from the system into the concentrate collecting tank, is started. At this instant the separation step starts. The flow rate of the concentrate let out into the concentrate collecting tank is taken with a flow meter in the outlet pipeline. To maintain the concentration level constant, the concentrate outlet rate shall be kept proportional to the rate of permeate outlet into the permeate collecting tank, as taken with the flow meter. The coefficient of proportionality shall be set at the beginning of the separation step. This constancy of the level of concentration shall be maintained throughout the liquid separation step, for which purpose the flow meters installed in the permeate outlet pipeline and in the concentrate outlet pipeline are connected to the respective pumps to control the flow rates of these pumps in accordance with the desired proportionality ratios. A suitable pump can be provided, e.g., by a piston pump operated by a proportional signal, which keeps the concentrate flow rate proportional to the permeate flow rate.
The hydraulic balance of the treatment system is properly maintained in the simplest way, using a level controller located in the tank for concentration of the liquid being treated. This regulator keeps the volume of the liquid being treated, which is added into the collecting tank, equal to a sum of the concentrate and permeate outlet rates.
Numerous sensors installed in the pipelines and tanks of the separation system keep track of the variation of the liquid characteristics, e.g., concentration, pH, temperature, pressure, conductivity, etc.
The liquid separation system can include some other components, e.g., an apparatus for membrane separation sterilization subsystem, a pre-filtration system for clarifying the liquid being treated, having its own flushing subsystem, a circulation loop which comprises a circulating pump and wherein the liquid is moved by the operating pressure equal to the apparatus for membrane separation pressure, a permeate suction pump, constant pressure controllers for maintaining the desired system operation pressure, automatic controls and a monitoring system. If necessary, the subsystem for pre-treatment of the liquid being treated can also be incorporated into the treatment system and it can comprise one or more precipitation, coagulation, adsorption, complexation steps. The sensors can all be connected to the programmable automatic controls of mechanisms (pumps, valves, motors) and the sensors themselves are controlled by the computer which can be used to set the control parameters.
According to the invention, the separation system is universal and it can be used for micro- ultra- and/or nano-filtration and for low-pressure reverse osmosis. In all of the aforementioned cases, no pre-treatment of the liquid being treated is needed, or little pre-treatment (coarse pre-filtration) can be done to prevent the selective layer of the membrane from being damaged with coarse particles. The said system can be mounted on a fixed platform; a mobile embodiment is also possible.
Another subject of the present invention is the process of separation of the liquid being treated into the permeate, or filtrate partially or fully devoid of substances not capable of passing through pores of the membrane, on the one hand, and the concentrate enriched with these substances, on the other hand; the aforementioned process comprises the following steps:
introducing the liquid being treated into the separation system comprising an apparatus for membrane separation, using the concentrating tank used to concentrate the liquid being treated and hence to prepare the liquid;
causing the impellers of each cell to rotate by increasing the apparatus for membrane separation pressure up to the operating differential pressure level;
dispensing the liquid into cells in the sequential or parallel manner by opening or closing of at least one liquid outlet means located in each intermediate cell in order to reach the desired level of concentration under the best thermal conditions for the said liquid;
optimizing the hydrostatic (pressure) and hydrodynamic [apparatus for membrane separation liquid flow rate, (rotary and radial) components of the speed in the operating clearance] parameters;
selecting the best oscillatory conditions (the oscillation amplitude and frequency) for the liquid in the apparatus for membrane separation from the standpoint of minimization of membrane clogging, which will be dependent on properties of these membranes and properties of the liquid being treated;
superposing vortex movements of the liquid in the operating clearance on oscillatory movement and on vibrational movements of the membranes in order to maximize the resistance of the membranes to clogging thereof;
separately receiving the permeate and concentrate for future use;
making the required settings, performing the required automatic switching, recording the separation system parameters and controlling these parameters.
The liquid is pumped under pressure into the first compartment of the apparatus for membrane separation, by at least one common liquid inlet means, filling each cell, including each operating clearance, and extracted by at least one common liquid outlet means. To create in the operating clearance the best conditions for generation of vortices preventing the membrane from clogging in every cell of the apparatus for membrane separation, the pre-selected ratio of the flow rate of the liquid let out from the first compartment of the apparatus for membrane separation to the flow rate of the said liquid let in into it shall preferably be maintained. The said ratio shall range between 0.05 and 0.99, preferably between 0.7 and 0.9.
According to the invention, various geometries of liquid circulation through the apparatus for membrane separation cells are possible.
The sequential flow of the liquid occurs when the said liquid enters the apparatus for membrane separation via the common liquid inlet means, passes through each cell of the apparatus for membrane separation and leaves via the common liquid outlet means located in the second end wall of the cylindrical housing. In this case the only purpose for which the peripheral liquid outlet means located in the ring-shaped wall of each cell, generally in its upper zone, are used is to bleed the air from the apparatus for membrane separation as it is filled with the liquid; in this case oscillations of the liquid flow are generated by the impellers located in the extreme cells, near the liquid inlet or outlet means.
The parallel flow of the liquid occurs when the said liquid enters the apparatus for membrane separation via the axial liquid inlet means of the hollow shaft having radial bores located at the level of each porous substrate of the apparatus for membrane separation. The liquid flows through the shaft and each radial bore and directly enters each cell. In the peripheral zone of each cell the liquid leaves the apparatus for membrane separation via the peripheral liquid outlet means. In this case the oscillations are generated by the impellers located in each cell of the apparatus for membrane separation.
The third possible case is a combination of bothxe2x80x94sequential and parallelxe2x80x94liquid passage methods. The bladed impellers make it possible to select between the sequential and parallel flows and change the ratio of one of the above flows to another in the cells of the apparatus for membrane separation.
According to the invention, the separation processes can be micro-filtration, ultra-filtration and/or nano-filtration, as well as low-pressure reverse osmosis. In all of the aforementioned processes no pre-treatment of the liquid being treated is needed, or little pre-treatment (coarse pre-filtration) can be done to prevent the selective layer of the membrane from being damaged with coarse particles.